The present invention relates to methods and apparatus for manufacturing hard-surfaced, cast iron articles such, for example, as rolls for use in the steel industry or paper calendering rolls, and the articles obtained by the method.
The manufacture of articles, such as rolls, utilizing casting in chill molds is a technique which is over 100 years old.
The particular method for manufacturing rolls and the associated metallurgical characteristics to be imparted to a roll during manufacture are determined by the particular application to which the roll is to be put. For example, rolls employed in rolling apparatus of the steel industry will substantially differ in these respects from calender rolls used in the paper making industry.
More particularly, conventional rolls used in the steel industry, such as in hot strip rolling mills, can be divided into two categories according to their use, namely, supporting and working rolls. The supporting rolls thus serve the purpose of providing support for the working rolls. Generally, there are usually two of each type of such rolls mounted in a single roll frame. The rolls can be manufactured substantially entirely of cast steel or can in some instances utilize spherical graphite iron. The supporting rolls may be manufactured utilizing socket shells formed as centrifugal castings, the socket shell being fixed upon a base shaft and being replaceable to increase the service life of the roll provided therewith.
The superficial layer of conventional working rolls provided at the initial end of a strip rolling line in the steel industry generally comprises a carbide having a martensitic-bainitic matrix structure. The microstructure of the central portion is generally pearlitic.
The most recently developed rolls used in modern strip roll mills are cast with the central portion thereof comprising cast iron. The mantle of the roll, however, is formed of iron which is alloyed with relatively large amounts of chromium. In this connection, the chromium content in the mantle portion of the roll is usually about 10 to 25%.
The casting of rolls for use in the steel industry as described above is generally accomplished by a static casting process. The manufacturing of high chromium rolls is generally performed using a centrifugal casting process. Thus, in the first steps of manufacturing a high chromium content roll, the high chromium alloy is poured into a rotating chill mold and in a latter step, the central portion of the roll is cast using common cast iron.
The raw material used for sheet rolls for use in the steel industry generally comprises steel or cast iron and has a chemical composition substantially the same as that of the strip roll described hereinabove. A static casting process is used for the manufacture of such rolls. In this connection, the outer surface of the roll is formed of an iron having a composition which differs from that used in the manufacture of the central portion of the roll so that these types of rolls are cast generally in the same manner as part of the strip rolls.
The rolling of profiled, structural steel as conventionally carried out can be generally divided into three steps, namely, rough, intermediate and finishing rolling. Spherical graphite rolls are conventionally used in the rough and intermediate rolling mills although rolls cast of steel are used to some extent. The finishing rolls generally comprise flaked, graphite rolls provided with an indefinite chill. The size of the rolls vary substantially depending upon the purpose to which the roll is put, the rough rolls comprising the largest size rolls having a hardness ranging from about 200 to 300 HB.
The rough rolls are generally cast in a sand mold followed by a normalizing treatment for homogenization.
Spherical graphite rolls are generally used in the intermediate rolling mills, such rolls having a hardness ranging from about 300 to 450 HB. Such rolls generally have a pearlite matrix, the hardness of which may be increased by suitably varying a portion of carbide which results in a carbidic structure which is determined by both the alloying and the effect of the chill mold cooling which occurs.
Rolls of the spherical graphite type may also be cast by a dual casting method by which it is possible to improve the mechanical characteristics of the central portion of the roll.
The material used for finishing rolls generally comprises either a flaked graphite or spherical graphite iron of the sharp boundary and indefinite chill type. The sharp boundary types are generally always cast by a dual casting process and are characterized by a graphite-free hard surface layer containing an abundance of primary carbides. The hardness of the hard surface layer can be suitably selected by on the one hand introducing carbidizing agents, (such as C,Cr,Mo) and on the other hand by influencing the matrix structure. In this connection, the matrix is generally pearlitic in the softer surfaced layers while being generally bainitic-martensitic in the harder surfaced layer. The structure of the rolls central portion is generally pearlitic with an abundance of graphite in either flaked or spherical form.
In the types of rolls formed by an indefinite chill, graphite will occur even in the surface layer thereof. For this reason, the thickness of the hard surfaced layer is difficult to determine thereby giving rise to the term "indefinite chill". However, as the hardness of the surface layer increases, the occurence of graphite becomes more scarce while the proportion of primary carbide becomes higher. This is achieved mainly through the variation of the content of chrome and silicon in the material.
The material used in the central portion of the rolls manufactured by the dual-pouring casting technique is usually soft grey cast iron. In cases where the roll is required to have a high mechanical strength, the material may also be cast of spherical graphite iron.
Turning now to calender rolls used in the paper making industry, such rolls are conventionally manufactured by using a single or direct casting process having sharp boundary characteristics. Thus, the surface layer of such rolls are generally free of graphite while the structure contains an abundance of primary carbide, the matrix including lower alloyed qualities of pearlite and higher alloyed qualities of bainite. At a certain depth below the surface of the roll, the portion of free graphite begins to increase, the material in the central portion of the roll already being soft and containing an abundance of graphite.
It is well known that calender rolls can be cast as solid rolls with an inner axial bore being subsequently formed. Alternatively, the axial bore may be formed using a core in the mold. It is generally considered today that a solid casting followed by a separate boring of the inner opening is superior as a manufacturing technique to the method in which the bore is formed during the casting operation.
Calender rolls have not been subjected to any heat treatment in the past and the casting technique utilized for manufacturing calender rolls is generally simpler than that used for manufacturing rolls used in the steel industry.
According to the prior art, the machining of rolls is accomplished in several working steps, the principle ones of which comprise rough turning, fine turning, rough grinding and then burnishing. The details of the particular steps, of course, depend upon the particular finish specification of the roll surface. Furthermore, journal pins of the rolls must be finished according to the particular usage to which the roll is put. Therefore, roll manufacturing facilities generally have at their disposal the following types of machine tools in numbers determined by production quantity and quality: rough and fine turning lathes, and rough grinding, burnishing, milling, drilling and boring machines. In addition, different types of machine tools are used when machining light, medium weight and heavy rolls.
Hard castings such, for example, as cast iron hard-surfaced rolls, are conventionally cast in permanent, metallic molds or chill molds. Chill molds have a relatively limited service life, only being usable an average of about three times. Since a number of different chill molds is required for a particular application, the cost of such molds represents a major capital investment.
The cast iron of which such hard-surfaced rolls are constituted comprises so-called white cast iron which, due to its hardness, is difficult to machine and, therefore, machining costs are quite high.